Puzzle to make you think

[mugsie]

I shoot reloads only, mine of course. So here's the question:
The other day I was shooting my .308 at 200 yards. 168g SMK over Varget. The load isn't important because it'll be different in your stick. They were grouping consistently at .63" for 5 shots (this was an average of multiple relays). The 168's will always shoot sub MOA.

One of the people next to me was evaluating commercial ammunition for her job. She asked me if I ever shoot commercial ammo and I told her never. She then offered me 5 rounds of FGMM, 168g also. I tried them and they grouped a few inches lower and about 3" to the right, but the group was also about .75" center to center.

What makes FGMM shoot so well in a wide variety of rifles (almost throws harmonics out the window) and why would one sequence group well at one location while another groups well but to the right? I can understand higher or lower, but to the right? Why lateral?

So I guess there's two questions, the secret behind FGMM and lateral impact.

Stay well people.

 

[notaglockfanboy]

It does throw harmonics out the window. It also tells me that all the guys who say seating depth matters more than anything are in a fantasy world. The only true answer can only come from the people behind the scenes at federal, and I doubt you will pry the secret out of them. Good luck in your quest.

 

[velocette]

Mugsie;
You can suspect: different powder, different powder charge, different primer, different neck tension, different case preparation (flash hole reamed?).
When I do load workups, ladder loads, I find that the POI changes up, down, left and right with the same bullet, same rifle, case, primer, but just different powder charges of the same propellant.
Rifles is like wimmen, they is all different and they all like different things. BUT Sierra Matchking 168s are like chocolate to wimmen, most all like it.

 

[WNTFW]

Well, I believe in the OCW method of load development, for the most part. What I don't buy into is that what works in one rifle necessarily works in another as far as the ultimate load goes. I do think there are certain rifles and loads that tend to work well in a specific range. IE the ballpark is in the same spot, but your favorite spot will vary. (notice I was careful no to use 'seat')

As far a seating depth I think many guys will jump to it when giving advice, without knowing any specifics. It depends on a few things. First is bullet type or more specifically ogive type. Next, is it even realistic to reach the lands? I don't think it is as much a priority as charge weight for the most part.

If you read this a couple of times some of it makes sense no matter what method you use. I think for me one take-away is use a method.
http://optimalchargeweight.embarqspace.com/

 

[rcmodel]

Barrel harmonics changes to POI dramatically when changing loads.

Both loads may be equally accurate.

But to groups will seldom if ever land in the same place.

The barrel acts like a buggy whip as the pressure and bullet friction race down it.

An accurate load allows the bullet to exit the muzzle crown in the same cycle of the whip rack time.

It just won't be in the same place in the cycle with too different loads.

rc

 

[earlthegoat2]

The reason any round groups accurately, even if at a different POI, is all about CONSISTENCY between rounds.

Apparently FGMM is consistent from round to round. When I say consistency I mean everything from the powder charge and bullet being within thousandths of a grain difference to the tooling marks on the case to the pressure of the crimp to the overall length to the ad nauseum.

This is nothing new to reloaders and accuracy junkies but for a factory to be able to crank out rounds like this borders on unrealistic to downright impossible.

How Federal does what it does as well as it does with the GMM is anybody's guess but I think that is the question you really need to ask the "people behind the scenes"

 

[jmr40]

I think it depends on the rifle and the individual bullet too. I have had some 308 and 30-06 rifles that would put 150's, 165's, and 180's so close at 100 yards to not matter. At longer ranges the lighter bullets would show predictably less drop with no change in windage.

With other rifles, or bullets, it is not uncommon to see some change in windage when different bullets are used. Sometimes even the elevation change is not what you expect either.

 

[mugsie]

Notaglock, I believe you're right, it's not harmonics at all but rather the consistency of the ammunition. The bullet doesn't care where on the vibration cycle it leaves the barrel as long as it leaves at the exact same spot each time. If that happens, then it will strike in the same spot each time too. It also demonstrates that vibration isn't only in the verticle plane but whips around in multiple planes sort of like a hose with water streaming out of it. As long as the exit point is the same, the strike point will remain the same as well.

That being said, then why don't all loads group well if the quality of the components is strictly controlled? Hmmmm? A conundrum.

 

[Snyper]

That being said, then why don't all loads group well if the quality of the components is strictly controlled? Hmmmm? A conundrum.

Because harmonics DO matter, and no two rifles no shooters are identical

 

[WNTFW]

The argument that harmonics do matter is this: You will have variations. Those variations will have a greater affect in the middle of a wave than at a peak of a wave.

It will make sense if you know how to properly toss a tennis ball to serve it.

 

[TEC]

Did you happen to shoot over a chronometer?

RC is correct about the harmonics/buggy whip and it follows that consistency of velocity is required to get the same POI. How else could might accurate rounds, at approximately the same velocities have such a different POI?

The tennis ball toss is a good analogy, too.

Comments about the seating depth and ogive are on point. SMK 168 gr has a tangent ogive, radius 2.215". Bullets with tangent ogive tend to be less sensitive to seating depth as the smooth taper of the tangent ogive to the point of contact of the lands and to the bearing surface of the bullet centers each round. Secant ogives are "finicky" about seating depth as they have a more abrupt transition from the circular arc of the bullet point to the bearing surface (but, typically, less drag and better BC's). Hybrid ogives (Berger) have a tangent ogive from the bearing surface to the point of contact with lands, then a transition to secant ogive to the bullet tip. Less sensitivity to seating depth; better BC (Reference: Litz, B. Applied Ballistics for Long-Range Shooting, 2nd Ed. P256-259.)

All else being equal -- bullet, powder, primer -- like the song, "It's all about the brass, 'bout the brass, 'bout the brass." Bullet ogives can only help so much. A poorly centered bullet relative to the bore, inconsistent brass neck length, neck thickness, and neck size all affect the tension holding the bullet and the "jump" of the bullet to the lands. A change in neck tension results in a change in timing of the actual instant the bullet starts out of the case and into the bore. Inconsistent timing of even milliseconds = inconsistent case pressure at ignition = inconsistent velocity = less accuracy. Ditto case volume consistency.

An alternate approach, albeit somewhat riskier, because of a more rapid rise in pressure at ignition, is to seat the bullet with relatively low tension, touching the lands or even longer, so that the bullet is jammed into the lands when the bolt is closed, and the bullet seats itself accordingly. In this case, the resistance of the pressure of the lands against the bullet's ogive centers the bullet to the lands and exceeds the effect of inconsistent neck tension. The tension to be overcome before the bullet starts down the bore is the the greater bullet tension against the lands than the bullet against the neck.

I'd bet, empirically, that if one put the Federal rounds to the test, Federal Match rounds have very consistent powder loads, case volumes, neck sizes, neck tension and concentricity. Combined with a good bullet that is insensitive to seating depth and there's the solution to the puzzle. Best way to test? Down range on the target. over a chrony.

 

[benzy2]

The other thing to notice is that these loads are very accurate, but we aren't talking guilt edge accurate. So they aren't perfect, but they seem consistently well made and as such, shoot consistently well (though probably not the absolute best for a particular rifle). I'd take it for what it is, well made factory ammo. Same thing happens in rimfire, just there is no reloading to compare against. A given option of ammo (typically relative to price) shoots well in nearly everything, though given lots and makes will shoot better or worse in a specific rifle. I think we put too much emphasis on some aspects of what makes accurate ammo, at least in terms of those short of dedicated bench rest shooters.

 

[Clint M]

I "think" Federal can use a crimp in place of you using seating depth to get consistent start pressure. Combine that with federal using quality components, and consistently assembling them it can amaze.

I have told this story to much I had a Bushmaster varmint ar15 that would shoot winchester white box 40 round varmint pack bullets .25" at 100 yards 10 shot groups. I wanted to hand load for this rifle I just couldn't do it... I don't see how factory ammo could shoot this good, and price at that time was as cheap as I could load my own with once fire brass.

 

[WNTFW]

Benzy,
You mention rimfire. In rimfire, where you can't handload, the benchrest guys tune the barrel to the load. Barrel tuners are quite prevalent. So while some think harmonics is at least inconvenient, it would seem there if evidence harmonics do matter. To me it shows that harmonics do come into play.

TEC,
I think it is not necessary to use a chrono to get good loads. However it sure can give some insight into what is happening. A friend of mine had some loads that would shoot to near perfection, EXCEPT for some stupid flyers. When chrono'ing each shot he made the correlation that the flyers were exceeding a given velocity. So it was either a harmonics thing OR the bullets didn't work well at a higher velocity, which is sometimes not common knowledge.
The chrono saved a lot of aggravation. He also had to be calling his shots but that is another deal.

Another big one for me is past a certain point group size is nice but in practical terms it only has to be good enough for the application and will vary with conditions. So as long as I can get consistently within a range I am good. .63 to .75 @ 200 is pretty good.

 

[WNTFW]

Come to think of if it The Browning BOSS system is tuning the barrel harmonics to the load as opposed to the load to the barrel.

 

[Ankeny]

I do think there are certain rifles and loads that tend to work well in a specific range. IE the ballpark is in the same spot, but your favorite spot will vary.

Exactly right. There are several examples, for instance: 6mmBR with 107 Sierra over 30.0 of Varget, 6.5 CRDM with a 120 A-Max over 44.5 of H4895 at 2.720 oal, etc.

 

[WNTFW]

Ankeny,
One of the main things that illustrates this to me is NRA HiPower competition. Most AR15s for SR can shoot a generic load well enough to not hinder the shooter. Most guys I know shooting a Rem 700 are within a couple of tenths on the powder charge. Probably the same with MR.

 

[joem1945]

I don't know what causes odd ball changes in POI but when I load two different manufactures of 69 gr bullets, one is sub min while the other is +1 min or more. The powder is the same lot, primers same lot and cases same head stamp. Puzzle for sure.